1. Field of the Invention
The present invention relates to an imaging apparatus and an imaging method that detect defective pixels of an imaging device and compensate image signals of the defective pixels using the detected results.
2. Description of the Related Art
An imaging device that is mounted in an electronic camera or the like and that images an object may have pixel defects that took place at the production stage. These pixel defects output signals, the levels of which are abnormal. Thus, if an image is generated with an output signal of an imaging device that has pixel defects, erroneous information that is absent in the object is contained in the image. As a result, the image becomes unnatural.
Such defective pixels may take place due to various causes such as a dirk current and a defective photodiode. A defect of which an output level of a defective pixel is higher than that of a normal pixel is referred to as “white defect,” whereas a defect of which an output level of a defective pixel is lower than that of a normal pixel is referred to as “black defect.”
As a method in which such defects especially white defects are detected and compensated, pixels that are light shielded and whose output levels exceed a predetermined value are detected and the positions thereof are stored in a storage device (that is a random access memory (RAM) or a read only memory (ROM)). When an object is imaged, the positions of the white defects are read from the storage device. The defective pixels are interpolated with their surrounding pixels. When a normal pixel is light shielded, the output level thereof is ideally zero. Using the difference of characteristics of a normal pixel and a defective pixel, the output level of which is not zero, a defective pixel is detected and compensated. This method has been proposed in the following Patent Document 1 and Patent Document 2.
[Patent Document 1]
Japanese Patent No. 2,565,264 (Japanese Patent Laid-Open Publication No. HEI 1-105672)
[Patent Document 2]
Japanese Patent No. 2,808,814 (Japanese Patent Laid-Open Publication No. HEI 3-296375)
In this case, detecting a defective pixel is recognizing a defective pixel as a defective pixel. On the other hand, compensating a defective pixel is compensating an output signal of a defective pixel that has been detected by an image process so that the output signal of the defective pixel becomes close to the expected output signal thereof.
The signal level of a defective pixel as a white defect varies depending on the time after the pixel is exposed until the signal is read therefrom (hereinafter this time may be referred to as exposure-read time). The signal level of the pixel is proportional to the exposure-read time. When the exposure-read time is short, the white defect is not obstructive. By contrast, when the exposure-read time is long, the white defect becomes obstructive.
However, in the foregoing defect detection method of the related art, pixels that output signals whose levels are larger than a predetermined value are detected as defective pixels. Thus, in all pixels to be checked (namely, individual pixels on one screen), their exposure-read times should be the same. In other words, as described above, the signal levels of defective pixels as white defects increase as their exposure-read times become long.
For example, when the exposure-read times of pixels at a lower portion of the screen are long and the exposure-read times of pixels at an upper portion of the screen are short, the signal levels of white defects at the lower portion of the screen are higher than those at the upper portion of the screen. At this point, if defective pixels are detected with a predetermined threshold value, even if defective pixels as white defects are almost equally distributed at the upper and lower portions of the screen, more defective pixels are detected from the lower portion of the screen than the upper portion of the screen. Thus, defective pixels may not been properly detected.